The present invention relates to an arrangement for controlling the flow of signals comprising a number of information packets in a communications network.
The invention also relates to a method of controlling the flow of signals in the form of information packets.
More particularly the invention relates to controlling the flow of information incoming to concentrating arrangements and rost particularly to switching arrangements in communications systems.
In digital communications systems of today the information is divided into packets. Each packet comprises a header and a data field. The header is the preamble of the packet and contains information about destination address, sometimes also originating address and control bits whereas the data part comprises the information intended for the given address. A cell is a short packet having a predetermined number of bits, i.e. a packet of a fixed length. Via switching arrangements in the communications network packets or cells are routed from various sources to destinations as given by the address information of the packets via packet switching arrangements. Switching arrangement which operate in so called synchronous transfer mode (STM) are known as well as other switching arrangements operating in asynchronous transfer mode (ATM). Some switching arrangements may also operate in both modes.
The ATM switching technique is a so called fast packet switching technique. This switching technique is particularly applicable when different sources have different requirements as to bandwidth. Via ATM different or mixed traffic streams such as for example data, voice, video or images can be transmitted asynchronously which is clearly advantageous in modern communication systems wherein one or more kinds of information may need to be transmitted.
In ATM, information is transported in the form of cells, i.e. packets of a fixed length. It is of great importance to the network how the packet switches operate. A number of cells may for instance arrive at a switch on a number of different input links at the same time and a number of them may have the same output link destination or there may be a discrepancy between the number of input links and the number of output links wherein the number of output links is considerably lower than the number of input links. This means that a number incoming cells may have to compete for an output link. The output link, however, cannot handle more than one cell at a time meaning that the other cells have to be stored temporarily in a buffer. This puts high demands on the buffering capacity and in some cases the capacity may not be sufficient and the cells may even run the risk-of being completely lost. It may also be difficult to fulfil the requirements as to waiting time etc.
Various buffering arrangements or buffering methods have been provided using input buffers, output buffers or a combination of both. One aspect relates to the head of the line problem (HOL). This is a problem that may occur when a number of data cells are stored in an input buffer which is a waiting position, i.e. the first cell in the buffer is waiting to be served. Then all data cells contained in that buffer have to wait, also those cells further down in the queue which are destined for other output links which for example at that moment might not be loaded at all. This means that neither the packet switch nor the output links are used to their full capacity but even very poorly. To solve these problems output buffers have been provided as well as intermediate buffers in a switch core etc. However, this may put high requirements on the output buffers and if there are a number of buffers at different locations, e.g. input buffers, intermediate buffers, output buffers etc. the switch gets very complicated. The copending patent application "Arrangement and method relating to packet switching" by the same applicant and filed at the same date as the present application discloses a way how to solve these problems and it is incorporated herein by reference.
It may also be required that different qualities of service, different QoS, are handled by the network. The switching arrangements as described in the above mentioned reference also deals with this.
There are different categories of signals one of which is CBR (constant bit rate). This category puts high requirements on the network and requires reservation of the needed bandwidth. This means that a network must have a high capacity for such signals since there are not accepted any variations in delay or delays as such at all. CBR signals are generally used for telephony and video signals. In U.S. Pat. No. 5,150,358 a system is given separating CBR signals from the others and handling them separately and giving them a higher priority. The CBR service class is however "homogenous" why the same problems are not encountered as when a service class is not homogenous.
Another category relates to signals of variable bit rate, VBR, which for example can be used for video. This relate to traffic streams having a guaranteed bandwidth but wherein the requirements as to delay variations are less strict.
A third category relates to ABR signals wherein ABR means available bit rate. The demands as to variations in delay are low or even none at all for these signals. What is important is that no cells are lost. The signals are e.g. used for data communication. There is today still no standardization relating to ATM ABR but such is expected to be produced soon. One controlling method based on cellrate measurements has been suggested. This means that switching arrangements in a network receiving ATM ABR signals may calculate a value on an adequate cellrate and feeding this information back to the sources of the signals. A source here means either a network terminating equipment or equipment within the network forming terminating equipment only as far as the flow control is concerned. However, since the method is based on measuring and feeding back of cellrate parameters, it works poorly when the cellrate is close to saturation. This means that links, for example expensive links, e.g. are not used fully or to an acceptable degree.
In WO 92/19060 an ATM switching arrangement is disclosed. Cells are divided into low loss and low delay cells respectively and the switching arrangement comprises a cell buffer which is divided into one memory area for each of the two types of cells. Depending on what kind of cell it is and on the buffer fullness, the cells are given different priorities for reading in and reading out respectively. Such an arrangement could however not in a satisfactory way deal with ABR signals.
U.S. Pat. No. 5,153,877 relates to allocation of the resources in a packet network. The resources are subdivided into subresources which are to be allocated to communications divided into different classes depending on QoS such as packet lossrate and transmission dElay. Also this document does not deal with resource allocation for ABR signals.
ABR signals for instance are particularly difficult to handle since irrespective of being of one and the same QoS, they can be of different categories such as being guaranteed resources to different extents. There may thus be a variation between not being guaranteed any resources at all up to being guaranteed resources to a considerable or a significant extent.